The venous blood from four species of freshwater fish, rainbow trout (Salmo gairdnerii Richardson), carp (Cyprinus carpio Linnaeus), bullhead (Ameiurus nebulosus Le Sueur) and sucker (Catostomus commersonnii Lacepède) was analyzed and an estimate made of the probable range of physiological CO2 tension. A detailed study was made of CO2 transport in the blood of two of these species, the rainbow trout and the carp, which differ greatly in the effect of CO2 on the combination of oxygen in the blood. They differ too, in their systems of CO2 transport. A curious feature of trout blood is the great change in packed cell volume with changes in the pressure of O2 and CO2. An hypothesis is presented to explain in part these anomalous changes in cell volume and other characteristics of the trout blood. Carp blood shows less differentiation from general vertebrate characteristics.